Hidden Features

Important and useful features/capabilities of a system that are available only if you know the secret code. Examples:

In MS Excel (at least through '98), F4 changes a cell reference from relative to absolute, cycling though all four possibilities. I have never found a menu entry or tool bar icon to invoke this action, and finding the process in the Help file is difficult. (As I recall, it is listed only under the topic that describes the actions of all function keys.)
Also in MS Excel, the powerful and useful array formula is hidden away unless you learn the ExcelCtrlEnterSecretHandshake.
To see just the most recent change to a wiki page, click on the date of that change at the bottom of the page or click on the time since the change as reported in QuickChanges. A neat and useful feature but hardly an intuitive user interface, or one that is documented in an obvious place. Both link to QuickDiff.
To see the links to a page, click on the title. Everybody knows about this, but it's not at all intuitive, and it took me ages to discover. -- TomAnderson
To see the last changes made on a wiki page, click on the date at the bottom of the page
Also - RightClick? on page to see what browser features are available for actions on the page.